allow to use reflection in constant time evaluation

[fdr400]

It is useful for constexpr switch cases, for example, using magic_enum::enum_switch

Example from our code base:

::parquet::schema::NodePtr makeNode(const ::reflection::Field& field) {
  const auto name = field.name()->str();

  return magic_enum::enum_switch(
      [&name](auto val) -> ::parquet::schema::NodePtr {
        constexpr auto base_type = val;
        if constexpr (::reflection::IsScalar(base_type)) { // <- must be constexpr to be evaluated
          return makeScalarSchema<ToCppNativeType<base_type>>(name);
        } else if constexpr (base_type == ::reflection::BaseType::String){
          return makeStringSchema<std::string>(name);
        } else {
          static_assert(false);
        }
      },
      field.type()->base_type());
}

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